The invention relates to the repair of damaged zones in intermediate layers of multilayer structures (or assemblies).
A “multilayer structure (or assembly)” is understood here to mean an assembly comprising at least one intermediate layer interposed between, and secured to, at least two other layers.
A cavity (or a through-hole) passes right through some multilayer structures of the abovementioned type, said cavity (or through-hole) having an approximately constant diameter and being able to comprise, in its immediate vicinity, in the region of one of its layers, a zone that is considered to be damaged since it contains at least one defect, for example a crack or a slit. This zone has to be removed and replaced with a new zone in order to avoid the damage from spreading, which could weaken the structure.
In the following text, a “cavity” is understood to mean an empty space passing right through a layer.
When the damaged zone is located around a cavity which is defined in an outer layer, the repair is carried out fairly easily by annular reaming of the damaged zone around the entire periphery of the cavity, followed by filling of the reamed (or removed) zone by means of one or more metal rings having a circular cylindrical shape, and finally securing of a nominal fastener (for example a screw).
When the damaged zone is located around a cavity which is defined in an intermediate layer, the repair is more complex on account of the fact that the damaged zone is interposed between undamaged similar zones. In this case, at least two repair techniques can be implemented.
A first technique consists in carrying out annular reaming of each of the layers of the structure around the entire periphery of their cavity, then either in filling the reamed (or removed) zone by means of one or more metal rings having a circular cylindrical shape, and in securing a nominal fastener, or in securing an oversized fastener.
A second technique consists in carrying out annular reaming only in the intermediate layer around the entire periphery of its cavity, then in filling the reamed (or removed) zone by means of an insert, and finally in securing a nominal or oversized fastener.
This latter insert is generally chosen depending on the profile of the reamed (or removed) zone that has to be filled. It may consist of a plurality of rigid metal parts which are introduced one after another into the reamed zone. However, this type of insert can bring about punctures (or pinched areas) in the reamed zone, and these can cause new defects.
As a variant, the insert may be a one-piece annular insert which is initially contracted at low temperature (for example by being immersed in liquid nitrogen). In this case, it may be produced from a composite material comprising for example minerals or an organic resin mixed with a powder or fibers or else a metal (for example lead). However, mineral-based composite materials are difficult to manufacture and thus expensive, and are poorly suited to the zones to be filled on account of their confinement, and the composite materials based on organic resin have low mechanical strengths and are thus unsuitable in the case in which the structure has to tolerate heavy loads and/or withstand high stresses.